Genome packaging is a key step in morphogenesis of large double-stranded DNA (dsDNA) viruses including tailed double-stranded DNA bacteriophages and herpesviruses, and is essential for assembly of infectious progeny virions. Genome packaging is a precisely coordinated molecular synergy, in which a certain amount of viral DNA from a DNA concatemer is inserted into a preformed procapsid, followed by binding of additional viral proteins to the capsid to retain the packaged DNA. This DNA insertion process is fulfilled by a powerful molecular machine consisting of the terminase and the portal. The portal forms a conduit at a single vertex of the capsid that allows viral DNA to enter during virus assembly and exit during infection. The terminase complex contains a DNA-recognition subunit that specifically binds to the viral DNA, and a catalytic subunit that provides the energy for the packaging reaction and cleaves the genome-length DNA from the concatemer. Molecular mechanisms of genome packaging in these large viruses are not well understood, owing to unusual complexity and lack of high resolution structural data. In particular, these genome-packaging proteins have to form high-order molecular assemblies in order to function. The portal protein forms a ring-like dodecamer embedded in the capsid. However, little is known about how the terminase assembles, how it assembles with the portal, and how the portal is embedded in the capsid. Our goal is to understand molecular mechanisms of genome packaging in DNA viruses by analyzing assemblies of genome-packaging proteins using structural approaches. We are particularly interested in the general question of how numerous proteins and other biological molecules, each present in multiple copies, assemble hierarchically into a sophisticated system to fulfill a complex biological process. In the present proposal, we seek to: (i) elucidate the high resolution structure of a complete, infectious bacterial virus; (ii) elucidate structures of component proteins of the terminase; (iii) elucidate the mode of DNA-binding for a terminase DNA-recognition protein; and (iv) explore the assembly of the terminase complex in vitro. Herpes viruses are important human pathogens that cause diseases ranging from chickenpox to various forms of cancer. This proposal is anticipated to contribute to public health by identification of novel targets of antiviral to control and prevent infection and diseases caused by herpes viruses.